Field of the Invention
The present invention relates to a light frequency control apparatus which is ideal for use, for example, in a frequency multiplex transmission system.
Relevant Art
In recent years, light frequency control devices for generating CW (continuous) light of a predetermined light frequency have been widely developed. A structural example of this type of device is shown in FIG. 4. As shown in this figure, a standard frequency light source 1 for outputting CW light 1a possessing a light frequency fixed at a predetermined value; light switch 2 for converting CW light 1 a supplied from standard frequency light source 1 into a light pulse signal 2a; light frequency shifter 18 for shifting and outputting a light pulse signal 2a supplied from light switch 2; and timing control circuit 17 for respectively supplying a timing signal 17a to light switch 2, and a timing signal 17b to light frequency shifter 18, are provided.
In light frequency shifter 18, a light coupler 4 for coupling and outputting light pulse signal 2a inputted via light switch 2 and a feedback signal supplied from frequency shifter 9 (to be explained hereafter); light amplifier 5 formed from EDFA (erbium addition optical fiber amplifier) for amplifying a light pulse signal supplied from light coupler 4; light coupler 6 for outputting a light pulse signal outputted from light amplifier 5 as light signal 6c, and for supplying a portion of this light pulse signal to delay optical fiber 7; delay optical fiber 7 for delaying a light pulse signal for a fixed delay period; polarization controller 8 for controlling the depolarization state of the light pulse signal supplied from delay optical fiber 7; and frequency shifter 9 for shifting the frequency of a light pulse signal, and supplying this resultant signal to light coupler 4 as a feedback light signal, are respectively provided.
According to this structure, light pulse signal 2a introduced from light coupler 4 circulates in a loop formed by means of light amplifier 5, light coupler 6, delay optical fiber 7, polarization controller 8, and frequency shifter 9 in light frequency shifter 18. Every time light pulse signal 2a circulates this loop, the frequency therein is shifted a fixed amount by means of frequency shifter 9, and delayed for a fixed delay period by means of delay optical fiber 7. As a result, a light signal 6c, in which the frequency component therein is shifted to form a staircase shape on the time axis, is outputted from light frequency shifter 18.
However, according to the conventional light frequency control device, in light frequency shifter 18, when the light pulse signal is amplified by means of light amplifier 5, even after the light pulse signal becomes zero (i.e., non-signal state), due to the properties of light amplifier 5, energy accumulates in the erbium dope optical fiber. Subsequently, when the next light pulse signal is supplied, the aforementioned accumulated energy is added thereto, which in turn, changes the peak characteristics of the pulse waveform to be outputted. In this manner, as shown in FIG. 5, the amplification of the peak portions becomes large and results in the output of a waveform in which the peaks are sharply distorted. In addition, the number of cycles of the light pulse signal in light frequency shifter 18, as well as the number of passages through light amplifier 5 both increase, resulting in an increase in the distortion of the above waveform. As a result, the deterioration of the S/N ratio of the light pulse signal increases, and the number of cycles therein must accordingly be reduced.